In multiradio concepts, where the number of different radio systems is increasing all the time, the interoperability of different radio systems is challenging. Different radio systems operating on different frequency bands are required to operate properly without disturbing each other, even if they are operating at the same time. This sets strict requirements for both receiver and transmitter chains especially in transceivers when a transmitter of a transceiver is having high power levels at the same time when a receiver of a transceiver is receiving a weak signal.
In the transmitter, a power amplifier is used to amplify the signal to be transmitted to the required power level. However, power amplifiers have usually a broad bandwidth. Therefore, they amplify the signal to be transmitted not only on the desired transmitter band but also outside the desired band. In such a case, an unwanted receiver band may be amplified with the same gain. After the power amplifier, there has to be a tight filter that filters these unwanted receiver band signals away before transmitting the power from the antenna. The requirements of such filters are strict causing losses also to the transmitter path. This, in turn, has to be compensated for with increased output power from the power amplifier. This decreases the total transmitter efficiency and leads to increased power consumption and increased heat in portable transmitters such as mobile phones.
Furthermore, the noise requirements for the transmitter path before power amplifier are very strict so as to guarantee that the noise level before the power amplifier will not be too high. This is required to ensure that the filters after the power amplifier can reduce the receiver band noise level to be low enough. If no filtering is performed before the power amplifier, the capability of the filters after the power amplifier sets a limit to the maximum gain of the power amplifier and can increase the required output power of the transceiver block before the power amplifier.
Traditionally, most of the unwanted noise filtering is performed after the power amplifier in a front-end module of the transmitter, which consists of switches and filters. Since the gain of the power amplifier is constant in both transmitter and receiver bands, it sets strict requirements for the filter after the power amplifier to decrease the signal level in receiver bands.
FIG. 1 illustrates a section of a traditional front end of a transmitter. The front end comprises a power amplifier 100 and a band pass filter 102 connected to the output of the power amplifier. The power amplifier comprises multiple amplifier stages 104, 106 and a matching circuit 108 after the stages. FIG. 2A illustrates an RF signal and noise strengths at the input of the power amplifier. Frequency is shown on the X-axis and signal strength is shown on the y-axis. A TX-arrow 200 denotes the signal strength on the desired transmission frequency. In addition, noise level 202 is shown. RX denotes the receiver band. FIG. 2B illustrates the RF signal strength at the output of the power amplifier. The RF-signal has been amplified on all frequency bands, as the power amplifier is a broadband amplifier. Both the desired transmission 202 and the noise signal 202 have been amplified. FIG. 2C illustrates the RF signal strength at the output of the band pass filter 102. The noise signal strength 202 on the RX band has been somewhat reduced.